Allowing SYSV shared memory to allocated more than 2GB

When trying to test PostgreSQL concurrency on Matt's Opteron 48 core box
with 64 GB of RAM I encountered the problem, that I wasn't able to set
kern.ipc.shmmax to more than 1,99 GB.

In colloboration with Sascha Wildner I created the following patch which
allows to address more >= 2GB of shared memory. I ran PostgreSQL 9.0.4
on a X86_64 VM w/ 4GB configured with shared_buffers set to 2 GB
(PostgreSQL uses shared memory for other things so we are well above the
previous 1,99 limit).

What isn't clear to Sascha and me: Do we need to bump the libc version
because of this? Opinions please.